1. Field of the Invention
The present invention relates to a system and method for managing a powertrain in a vehicle.
2. Background Art
Four-wheel-drive vehicles—conventional or hybrid electric vehicles (HEV's)—utilize various driveline components to deliver propulsion torque to the wheels of the vehicle. For example, when a part-time four-wheel-drive system is operated in the four-wheel-drive mode, torque is delivered continuously to both a primary axle and a secondary axle. While each axle contains a differential, the part-time system has no center differential—i.e., there is no differential between the two axles. This means that if one of the axle's wheels are on a zero-traction surface torque is still deliverable to the other axle's wheels.
The absence of a center differential improves capability off-road, but precludes on-road usage because the lack of a center differential can cause problems with stability, handling and steering, and tire scrub. Many part-time four-wheel-drive systems utilize wheel-end disconnects so that the wheels on the secondary axle can be disconnected from the driveline when the vehicle is in the two-wheel-drive mode. Disconnecting the wheels, in conjunction with disconnecting the secondary axle from the primary axle at the transfer case, means that the secondary axle components do not rotate. This removes the drag of the secondary axle components, and increases fuel economy.
Another type of four-wheel-drive system is a full-time system, that is typically meant for on-road and light off-road usage. A full-time four-wheel-drive system distributes torque to all four wheels continuously. Unlike its part-time counterpart, it provides a differential in the transfer case to allow differentiation between the axles, making it fit for on-road use. A full-time system is often considered less capable than a part-time system in four-wheel-drive mode, because of its center differential. Unless the center differential has a torque biasing device, it acts as an open differential that delivers no torque to any of the four wheels, when one of the wheels is on a zero-traction surface.
An on-demand four-wheel-drive system, used for on-road and light off-road purposes, seeks to combine the advantages of both part-time and full-time systems by delivering torque intermittently to the secondary axle via a friction clutch. The intermittent transfer of torque to the secondary axle, if controlled properly, behaves like a center differential during on-road use, while the application of the friction clutch simulates a part-time four-wheel-drive mode providing increased traction for off-road use. While the on-demand system can make the vehicle capable in both on-road and off-road uses, it lacks the fuel-saving benefits provided by the wheel-end disconnects. The rapid application and release of the friction clutch in the transfer case precludes the use of wheel-end disconnects, which cannot provide fast and frequent engagement and disengagement. Therefore, a need exists for a system and method that provides the advantages of an on-demand four-wheel-drive system, with the fuel economy benefits of a part-time system.